Fix XCode 11.3+ build problem
Fixes #97
Fixes problem building when using XCode versions 11.3 or 11.4.
Fixed by upgrading to Stripe iOS SDK v19.0.
This required changing the minimum supported iOS platform version from v9 to v10.
Although the API is compatible with v6.2.1, the minor version was updated because of the change in minimum supported iOS platform version.